Confined spaces such as building interiors may have limited exchange of moisture vapor with adjacent or external environmental spaces. Materials having moisture permeance that is variable when exposed to different ambient conditions can provide benefits for construction of energy efficient buildings.
Buildings are commonly equipped with thermal insulation in order to provide comfortable living conditions inside the building. In addition to insulation, buildings are also constructed with various air barrier materials to limit air and water infiltration into the structure of the building. These air barriers minimize heat loss through draft flow.
In addition to air barriers, vapor barriers have also been used for controlling flow of moisture. For example, the moisture flow is generally from inside to outside when the exterior air is cold and dry (winter) and from outside to inside when the air is hot and humid (summer). In regions where these conditions prevail, it is common practice to use a vapor barrier such as polyethylene (PE) film on the warm side of the insulation, generally behind the gypsum wallboard, to prevent moisture diffusion from the interior heated space to the exterior cold environment. Such vapor barriers reduce the potential for moisture condensation in the interior wall cavity space as the water vapor is exposed to the cold exterior temperatures. Although the use of PE film as a vapor barrier reduces moisture condensation during exterior conditions of cold temperatures and low humidity, the same vapor barrier increases the likelihood of moisture condensation at the insulation-PE film interface during hot, humid summer conditions where moisture flow is from outside to inside. Therefore, it is desirable to have a material that acts as a vapor barrier during winter but facilitates vapor permeation during hot, humid summer conditions. It is also desirable to have a vapor barrier that can become vapor permeable under humid conditions in the wall cavity caused by unexpected uncontrolled events like water leaks into the wall cavity, to facilitate drying of the wall cavity by inward moisture flow as well as outward flow. Under conditions of low ambient humidity, low moisture permeance is desirable and under high ambient humidity, high moisture permeance is desirable.
Besides having desirable vapor diffusion properties, a vapor barrier article also requires having other properties to form in the conditions in the roof. The vapor diffusion properties should remain stable in long duration over the usage conditions. Thus an article should be thermally stable, and have good mechanical properties and anti-static properties. The material is also preferred to be environmentally safe, such as not emitting toxic gas in the event of fire. The material should be compatible with other materials, especially with those of which other layers of a multilayer structure are made. This relates to the ease of preparation of the article, the mechanical integrity of the article, and associated cost.
U.S. Pat. Nos. 6,808,772; 6,878,455; 6,890,666 and 7,008,890 disclose applications of a polyamide building liner material that has a water vapor diffusion resistance (WVDR) of from 2 to 5 meters diffusion-equivalent air space width at a relative humidity (RH) of an atmosphere surrounding the vapor barrier between 30% and 50% and a WVDR of less than 1 meter diffusion-equivalent air space width at a relative humidity between 60% and 80%. See also US Patent Application Publication 2003/0215609 (a moisture-adaptive vapor-barrier film comprising PE and acrylic ester with a WVDR of from 0.5 to 100 meters diffusion-equivalent air space width at an RH of an atmosphere surrounding the vapor barrier between 60% and 80%) and U.S. Pat. Nos. 7,442,659 and 7,914,634 (use of ionomers for sealing insulating materials having a WVDR of from 1 to 20 meters diffusion-equivalent air space width at an RH of an atmosphere surrounding the vapor barrier of 25% and a WVDR of from 0.02 to 0.7 meter diffusion-equivalent air space width at a relative humidity of 72.5%).
U.S. Pat. No. 7,829,197 discloses a film or sheet comprising a neutralized acid copolymer or ionomer thereof, an organic acid, and optionally other polymers, having a ratio of wet cup permeance to dry cup permeance greater than 10. U.S. Pat. No. 7,838,104 discloses a multilayer film or sheet which comprises at least one vapor barrier layer and at least one vapor transmission-adjusting layer wherein the vapor transmission-adjusting layer comprises a neutralized acid copolymer or ionomer thereof, an organic acid, and optionally other polymers, having a ratio of wet cup permeance to dry cup permeance greater than 10.
There is a continuing need for a new material with vapor diffusion properties that can be tailored to meet varied building requirement in varied geographical conditions.